Enzymes long have been recognized as stellar performers in the cosmos of life. As catalysts their efficiency and specificity are unmatched by man-made materials. As molecules their structure often is capable of a multitude of permutations to permit adaptation for functionality in differing species or on differing substrates. Although man has utilized enzymes to effect desired changes at least since the inception of recorded history (for example, in the production of fermented beverages) it is only relatively recently that he has achieved the first glimmerings of understanding how enzymes work.
That enzymes are produced only in living systems imposes various distinct limitations on their commercial use. For example, one such limitation is their production and supply; because they are efficient, enzymes usually are produced only in small amounts. If one desires to isolate a purified enzyme the situation is exacerbated since living systems generally secrete a large number of enzymes, so it is necessary to isolate a small quantity of one enzyme from a host of structurally related materials of a different functionality. Another limitation is that use of an enzyme away from living systems often is difficult. In some cases this is not a serious limitation because the fermentation process is quite acceptable. More recently this limitation has been diminished using immobilized whole cells or the immobilized enzymes themselves to effect the desired chemical change. Yet another limitation is the enzymes' susceptibility to activity loss, generally by denaturation of the polypeptide or protein portion characteristic of enzymes. These limitations, among others, have provided an impetus to construct molecules which manifest the beneficial attributes of enzymes to the exclusion of their undesirable attributes.
Some enzyme systems require cofactors, or coenzymes, for the system to be operative. In a sense such systems have the additional limitation of requiring both an enzyme and its cofactor for the process to be carried out. In another sense such a system may provide the opportunity to use the cofactor, or an analogue, apart from the enzyme in a system where the enzyme's role can be readily played by a chemical reagent. Whether it be the synthesis of a molecule showing enzyme properties or a molecule acting as a coenzyme, there is great motivation to construct mimics of biological catalysts.
This application relates to the preparation and use of enzyme mimics. In a narrower aspect it relates to enzyme analogoue polymers, i.e., polymers operationally analogous to enzymes or coenzymes and which may, as in this case, be structurally analogous as well. More specifically, we are here concerned with polymers which are mimics of nicotinamide adenosine diphosphate and the use of the reduced form of such mimics in asymmetric reduction of carbonyl, thiocarbonyl, and imino groups in organic materials to form chiral alcohols, thiols, and amines, respectively.
Nicotinamide adenosine diphosphate (NAD) is a coenzyme for many dehydrogenases. NAD is readily reduced, the reduced form generally being abbreviated as NADH.sub.2, and the NAD--NADH.sub.2 pair acts as a redox couple for many substrates. This is schematically depicted below, where SH.sub.2 and S are the reduced and oxidized forms, respectively, of a suitable substrate. ##STR1## That NADH.sub.2 is the 1,4-reduction product of the nicotinamide portion of NAD is well accepted; thus, nicotinamide is the "active site" of the coenzyme.
The polymers of this invention are analogues of NAD and mimic the NAD--NADH.sub.2 couple. The polymers are functional analogues in readily shuttling between the unreduced and reduced forms. The polymers are structural analogues in containing modified nicotinamide as a recurring functional unit. The polymers are not analogues of NAD in that the reduced form of the polymer may be prepared in the absence of an enzyme, dehydrogenase, and oxidation of the reduced form of the polymer, i.e., hydrogen transfer to a substrate, also occurs in the absence of an enzyme.
The specificity of enzyme catalyzed reactions previously has been alluded to, and such specificity extends to stereospecificity in the sense that only one of many enantiomers of a chiral material may be a suitable substrate, or only one of several enantiomers may be the product of enzyme catalyzed transformation. Such stereospecificity extends to NAD--NADH.sub.2 where two D-ribose units provide an asymmetric environment. In this invention we have provided polymeric enzyme analogues with an asymmetric environment in close proximity to the amide nitrogen of the nicotinamide and nicotinamide-like recurring units, with the result that the reduced form of the polymer effects asymmetric reduction of suitable substrates, including the carbonyl group of aldehydes, ketones, and .alpha.-ketoesters, the thiocarbonyl group of thioaldehydes, thioketones, and .alpha.-thioketoesters, and the imino group of imines and .alpha.-iminoesters. In short, the polymers described herein are mimics of the NAD--NADH.sub.2, system where the reduced form serves as a chiral pseudohydrogenase for substrates containing the aforementioned functional groups.